Electrophotographic process using a high intensity electromagnetic radiation source

ABSTRACT

A METHOD FOR FORMING AN ELECTROSTATIC LATENT IMAGE ON THE SURFACE OF A NONPHOTOCONDUCTIVE DIELECTRIC MATERIAL IS DISCLOSED. IT HAS BEEN DETERMINED THAT THE CHARGE ON THE SURFACE OF AN INSULATING MATERIAL MAY BE SELECTIVELY DISSIPATED, ALTHOUGH THE MATERIAL ITSELF IS NOT PHOTOCONDUCTIVE, UPON EXPOSURE TO A HIGH INTENSITY ELECTROMAGNETIC RADIATION SOURCE.

1 D H. PERRY 3,561,957

' ELECTROPHOTOGRAPHIC 'PRocEss USING A HIGH INIENSITY ELECTROMAGNETICRADIATION SOURCE Filed Sept. 23, 1966 INVENTOR. DAVID H. PERRY A TTOR/V5 Y United States Patent US. Cl. 96-1 2 Claims ABSTRACT OF THEDISCLOSURE A method for forming an electrostatic latent image on thesurface of a nonphotoconductive dielectric material is disclosed. It hasbeen determined that the charge on the surface of an insulating materialmay be selectively dissipated, although the material itself is notphotoconductive, upon exposure to a high intensity electromagneticradiation source.

This invention relates to an imaging system and more specifically to aphotoconductorless imaging system.

Many image and signal recording processes are known which involve thecreation of an electrostatic record from or by means of which a visiblerecord may subsequently be formed. One such technique, a form ofelectrostatic electrophotography, otherwise known as xerography, employsthe use of a photoconductive insulating medium to form latentelectrostatic images with the aid of electromagnetic radiation toproduce a visible record. A second image forming process known aselectrothermography relies upon a phenomona wherein certain dielectriclayers, when charged electrostatically and heated exhibit breaks intheir respective charge-temperature curves at a critical temperature, atwhich point the material experiences a sharp increase in conductivity.When exposed to heat in an imagewise configuration and the temperatureexceeds the critical point for the particular plastic, the originalcharge is selectively dissipated in the heated, lower resistant areas.By dusting the remaining latent electrostatic image with a developer ofthe same charge reversal development will occur in the now unchargedareas and a positive copy obtained. A third imaging system useful forrecording information termed chemography has been developed whereinirreversible chemical changes induced by light in specific materials cangenerate a persistant conductivity pattern which may be used forelectrostatic imaging. Exposure to light in the absorption region forthe particular material renders the material much more electricallyconductive than before exposure During or after exposure the film issufiiciently conducting so that, if an electrostatic charge is applied,it leaks off to ground in the exposed areas leaving an electrostaticlatent image capable of visible development.

While these and other techniques have been found useful as image andsignal recording processes there are inherent disadvantages to theiruse. For example, in the case of the xerographic process, it isnecessary in order to develop and record specific information that aphotoconductive material be utilized either alone or in combination witha particular binder material. A second disadvantage to this system isthat it is required that the photoconductive material have the necessaryresistivity properties so that it may support an electrostatic charge inthe dark. The electrothermographic process is generally limited by thematerials which will respond to and absorb the heat necessary to satisfythe requirements of the system in reproducing a copy of the original. Asecond disadvantage to this system is that specific dielectrics must bechosen which will exhibit the necessarycharge-tem- 3,561,957 PatentedFeb. 9, 1971 perature response required. In the chemographic process itis again necessary to select a specific material which is bothphotosensitive and will have its electrical conductivity altered uponexposure to light.

It is, therefore, an object of this invention to provide an imageforming system which will overcome the above noted disadvantages.

It is a further object of this invention to provide a novel method ofpreparing a photographic reproduction.

Another object of this invention is to provide a novel method ofproducing an electrostatic latent image.

Still a further object of this invention is to provide a novel imagingsystem which is generally not limited by the materials of the process,such as a requirement to use a photoconductive or photosensitivecomposition.

The foregoing objects and others are accomplished in accordance withthis invention, generally speaking, by providing an imaging systemwhereby a support substrate such as aluminum, is coated with adielectric material, such as polyethylene terephthalate, which willretain on the surface thereof an electrostatic charge when subjected toa charging mechanism. The charged surface of the resulting dielectricplate is selectively exposed to a high intensity electromagneticradiation source, such as, for example, a xenon flash tube, for apreferred period of time of from about 60 to about microseconds, therebydepleting the charge in the exposed areas and producing an electrostaticlatent image on the surface of the dielectric material. The resultinglatent image is developed thereby producing a copy of the originaldocument.

It has been determined that the charge developed on the surface of adielectric material may be selectively dissipated, when exposed to ahigh intensity electromagnetic radiation source, in a very shortinterval of time. Although it has further been determined that a lowenergy radiation source will produce a similar result when applied tothe surface of a charged dielectric for an increased period of time soas to ultimately expose the charged surface to an equivalent amount ofenergy as produced by the high energy source, such an exposure becomesquite impractical and essentially non-useful as an image and signalrecording process. Therefore, in order to effectively utilize the chargeremoval mechanism of the present invention as an image recording systemit is generally considered desirable to employ a high intensitypulsating or flash radiation source which will selectively deplete thecharge on an insulating layer in a corresponding limited time interval.In order to achieve the desired results, it has been determined thathigh energy pulsating sources such as flash lamps containing xenon,krypton, helium, neon, and/or argon gases and certain discharge devicescontaining gas combinations such as iodine-xenon and/or kryptongenerally provide the intense, concentrated electromagnetic radiationnecessary. Although energy sources producing light in a continuouselectromagnetic spectrum, such as xenon arc lamps or incandescent lampswhich initially produce a lower energy quantum, may be used in order toobtain .the required energy level, it is necessary to increase theexposure time to such a degree so as to make the process impractical. Asa result of the exposure of the charged surface of the dielectricmaterial to a high energy pulse, a charge differential is developedbetween the exposed and non-exposed areas, due to the decay of thecharge in the exposed areas, in a very limited time interval therebyallowing for the development of a recognizable image.

In accordance with the present invention the surface of an insulatingmaterial, such as polyethylene terephthalate, is charged to anelectrostatic potential by any suitable technique, such as by a coronacharger. The

charged surface is selectively exposed to a high energy flash lamp, suchas a xenon flash tube which may be commercially obtained from PEK Labs,-Inc., Sunnyvale, Calif, thereby exposing the charge surface of thedielectric to a concentrated energy source ranging from about 200 toabout 1000 joules input. Optimum results were obtained when theenergy'level was maintained between about 4-00 to about 1000 joules,thereby more consistentlyestablishing the most desirable voltagedifferentials between the image and non-image areas. The imagingcharacteristics of the present invention may be obtained by establishinga voltage differential between the image and non-image areas of about 20volts with optimum results being obtained at a voltage differential ofabout 40 voltsor more. To produce a developable image it was determinedthat, as a result of the exposure to the above mentioned energy quantum,a voltage differential of 20 volts or more could readily be obtained.

The effective time interval for providing the desired energy input asprovided by the pulsating radiation sources ranges from about to about140 microseconds with the major portion of the energy being suppliedduring a period of between about 20 to about 120 microseconds. However,the period of time at which the major portion of the discharge willoccur will vary depending upon the specific energy source that is beingemployed. It is during this period that the maximum efficiency of thepulse generated by the radiation source is realized. In addition to theeffect of the amount of energy incident upon the charged surface of thedielectric is the effect of distance which exists between the source ofthe radiant energy and the surface to be exposed. The amount of thecharge that is dissipated will vary depending upon this distance, therebeing a greater differential produced as the distance between the energysource and the surface exposed is shortened. For purposes of the presentinven- 'tion this distance is generally restricted to from about '3 toabout 12 inches with a preferred range being from about 5 to about 7inches to obtain maximum eifciency of the imaging system.

Any suitable electromagnetic radiation source may be used in the courseof the present invention. When used in a manner as specified in theabove discussion, any similar energy source will produce the dischargeeffect necessary. Typical electromagnetic radiation sources are xenonflash lamps, helium, neon, argon, and krypton filled ilash lamps, andcombination iodine-xenon and/or krypton filled electric space dischargedevices. Exploding wires are also a source of high energyelectromagnetic radiation. Xenon arc lamps and incandescent lamps areexamples of continuous radiation sources which ultimately will producethe effect desired by the present invention. In. order to effectivelyutilize the charge removal mechanism of the present invention as an,electrographic recording system it is preferred to use a high intensitypulsating or flash energy source to record the image in a limited periodof time. As a result of the limited accessibility to the preferredenergy sources, it is expedient to use the xenon and krypton filledflash lamps.

Any suitable insulating or dielectric material may be used in the courseof the present invention such that it will retain an electrostaticcharge when so exposed. Typical insulating materials are the Staybeliteresins, a family of thermoplastic synthetic resins prepared fromhydrogenated rosin and commercially available from the Hercules PowderCo.; polyethylene terephthalate ('Mylar); styrene polymers such asVelsicol, a styrene terpolymer commercially available from the VelsicolChemical Corp. and Piccolastic resins, styrene polymers available fromthe Pennsylvania Industrial Chemical Corp.; ethyl cellulose; celluloseacetate; polycarbonates such as Plestar commercially available fromGeneral Aniline and Film Co.; polyethylene; polypropylene; polymericPowder Co.; various thermosetting resins such as phenol formaldehyde,urea formaldehyde, and epoxy resins; silicone resins; VYNS, a vinylchloride-acetate resin commercially available from Union Carbide Co.';polyvinyl chloride; Nevillac, an alkyl hydroxy resin commerciallyavailable from Neville Chemical Co.; Tedlar, a polyvinylfluoride resin,commercially available from E. I. du Pont de Nemours and Co.; glass; andnumerous electrographic papers, such as C62-145, commercially availablefrom Crown Zellerbach, and 9510-9, commercially available from CrockerHamilton. Optimum results were obtained when using the electrographicpaper commercially available from Crown Zellerbach and Mylar, apolyethylene terephthalate film commercially available from E. I. duPout de Nemours and Co.

Any suitable toner or developer may be used in the course of thisinvention such as those disclosed in US. Patents 2,788,288; 3,079,342and Re. Patent 25,136. The toner is generally a resinous material whichwhen fixed has the necessary properties which will retain ordinaryprinting inks. Typical developer powders are styrene polymers, includingsubstituted styrenes such as the Piecolastic resins commerciallyavailable from the Pennsylvania Chemical Corporation, phenolformaldehyde resins, as well as other resins having similar properties.The developer powder or electroscopic marking particles may be admixedwith a carrier such as glass beads. The toner the insulating material orthe developing particles may be admixed wth a carrier such as glassbeads. The toner may be applied in the form of a mixture with magneticparticles, such as magnetic, iron, to impart a charge to the developerpowder particles tribo-electrically. A developer particle is so chosenthat it is attracted electrostatical- 1y to the charged image and/orrepelled from the background area to the charged image and held thereonby electrostatic attraction.

Liquid developers may also be used in the course of this invention.Typical liquid developers are disclosed in US. Patents 2,890,174 and2,899,335. Generally, the developer comprises a liquid combination ofmutually compatible ingredients, which when brought into contact with anelectrostatic latent image, will deposit upon the surface of the imagein an image-wise configuration. In its simplest form, the compositioncomprises a finely divided opaque powder, a high resistance liquid andan ingredient to prevent agglomeration. Typical high resistant liquidsare carbon tetrachloride, kerosene, benzene, trichloroethylene and othersuitable hydrocarbons having a boilingpoint between about 70 and 200 C.Any of the finely divided opaque solid materials known in the art suchas carbon black, talcum powder, and other pigments may be used in theliquid developer. Silica aerogel, commercially available from MonosanoChemicalCo, is a deagglomeration ingredient generally used. However, anyconventional and/ or suitable ingredient known to be useful by the priorart in a liquid development system may be utilized. In addition, othercommercially available oil based inks including alcohol base inks may beused in connection with this invention.

In addition'to using the liquid developers containing a high resistancecomponent suitable aqueous based or polar liquid developers may also beused when the system so warrants. Furthermore, any conventional and/ orsuitable aqueous based ink may be used in the process of the presentinvention either alone or in conjunction with the above developer,particularly when the developer is colorless. This includes both theinks containing a water soluble dye substance and the pigmented inks. Itis also possible to use simple compounds or color water solublecomplexes, such as those which are formed by certain number oftransition elements. Typical such compounds are the known cuproustetramine complex, chromium salts such as chromium sulfate, chrome andpotassium alum, potassium chromate, the aquo-and aceto-complexes oftrivalent chromium, certain ferric compounds such as ferric thiocyanate,the prussic compounds of iron, iron-ammonium citrate, the thiocyanateand thiocyanocobaltates of bivalent cobalt, cobalt sulfate and chloride,the chlorides and sulfates of bivalent nickel, the ferric complexes withsalicylic acid, and the compounds formed between titanium and ironsalts.

The invention is illustrated in the accompanying drawing in which theexemplary electrostatic copying apparatus utilizing the process of thepresent invention is illustrated.

An exemplary electrostatic copying apparatus adapted to employ theprocess of the present invention in the form of a cylindrical drum isillustrated. The drum, generally designated 14 comprising a dielectricmaterial 15 coated on support substrate 16, when in operation, isgenerally rotated at a uniform velocity in the direction indicated bythe arrow so after portions of the drum periphery pass the charging unit18 and have been uniformly charged they come beneath the energy source19 for exposing the charged surface to the image to be reproduced.Subsequent to charging and exposure, sections of the drum surface movepast the developing unit, generally designated 21. This unit is of thecascade type which includes an outer container or cover 22 with a troughat the bottom containing a supply of developing material 23. Thedeveloping material is picked up from the bottom of the container andcascaded over the drum surface by a number of buckets 24 on an endlessdriven conveyor belt 26. This development technique which is more fullydescribed in US. Patents 2,618,551 and 2,618,552 utilizes a two elementdeveloping mixture including finely divided-pigmented marking particlesor toner and larger carrier beads. The carrier beads serve both todeagglomerate the fine toner particles for easier feeding and chargethem by virtue of the relative positions of the toner and carriermaterial in the triboelectric series. The carrier beads with tonerparticles clinging to them are cascaded over the drum surface.Electrostatic field from the charge pattern on the drum attracts tonerparticles from the carrier beads serving to develop the image. Thecarrier beads, along with any residual toner particles not utilizedduring the development step fall back into the bottom of the container22. The developed image continues around until it comes into contactwith the copy web 27 which is passed up against the drum surface by twoidler rollers 28 so that the web moves at the same speed as theperiphery of the drum. The toner in the developing mixture isperiodically replenished from a toner dispenser not shown. A transferunit 29 is placed behind the web and spaced slightly from it between therollers 28. This unit is similar in nature to the surface chargingmechanism 18 in that both operate on the corona discharge principal.Both the charging device 18 and the transfer unit 29 are connected to asource of high DC potential of the same polarity identified as 31 and32, respectively, and including corotron discharge wires represented at33 and 34, respectively, surrounded by a conductive metal shield.

In the case of the corona discharge transfer unit, a charge is depositedon the back ofweb 27 and this charge is of the same polarity as thecharge initially deposited on the drum and also opposite in polarity tothe toner particles utilized in developing the latent image. A dischargedeposit on the back web 27 pulls the toner particles away from the drumby overcoming the force of attraction between the particles and thecharge on the drum. It should be noted at this point that many othertransfer techniques may be utilized in conjunction with the presentinvention. For example, a roller connected to a high potential sourceopposite in polarity to the toner particles may be place immediatelybehind the copy web or the copy web itself may be adhesive to the tonerparticles. After transfer of the toner image to web 27, the web movesbeneath a fixing unit 36 which serves to fuse or permanently fix thetoner image to web 27. In this case, a resistance heating-type fixer isillustrated. However, here again, other techniques known in the art mayalso be utilized including the subjection of the toner image to asolvent vapor or spraying of the toner image with an adhesivefilm-forming overcoating. After fixing, the web is rewound on a coil 37for later use.

After passing the transfer station, the drum continues around and movesbeneath the cleaning brush 38 which prepares the surface for a new cycleof operation. It should be noted that this apparatus may also beoperated at varying speeds by setting the corona discharge unit at ahigh enough voltage so that the surface of the dielectric will becharged fully at the highest speed. Overcharging Will not occur becausethe drum configuration is of such a nature and has such electricalcharacteristics that any excessive current will be drained away by itsvoltage regulating characteristics.

Although the invention has been described in connection with coronacharging, it is to be understood that this is exemplary only, and thatthe self-regulating drum may, in fact, be employed with any suitablecharging technique. Other suitable charging methods include chargingwith a scorotron unit, friction charging and induction charging asdescribed in US. Patent 2,934,649 and 2,833,930 and roller charging asdescribed in US. Patent 2,934,650. Furthermore, any suitable developmentmeans may be used in the course of this invention such as cascadedevelopment as described in the illustration or powder cloud developmentmore fully described in US. Patent 2,725,305 and 2,918,910 and magneticbrush development more fully described in US. Patent 2,791,949 and3,015,305. Furthermore, as mentioned above, liquid developers may beused to develop the latent image formed on the surface of the dielectricmaterial in which case the, liquid may be applied to the surface of thedrum in the form of a continuous film, as an ink mist generated with anultrasonic unit as more fully described in US. patent application Ser.No. 418,642, or the electrostatic latent image may be developedutilizing a gravure dispensing unit according to the process describedin US. Patent 3,048,043.

To further define the specifics of the present invention, the followingexamples are intended to illustrate and not limit the particulars of thepresent system. Parts and percentages are by weight unless other-wiseindicated. The examples are also intended to illustrate variouspreferred embodiments of the present invention.

EXAMPLE I Crown Zellerbach C-62-l45 electrographic paper is charged on aJossel designed scorotron charged at about +5000 volts DC at aboutmicroamps charging current. Five passes by the scorotron charger aremade. The charged surface of the electrographic paper is selectivelyexposed by way of an image stencil for approximately 80 microseconds toa PEK xenon flash lamp fired at about 3550 volts DC with about 136microfarads of capacitance. The distance between the flash lamp and theexposed surface is maintained at about 6 inches. The number of joulesincident on the surface of the exposed dielectric paper is measured atabout 1000 joules, approximately 0.1 joule per square centimeter. Theelectrostatic latent image thereby produced is developed according tothe magnetic brush technique as disclosed in US. Patent 2,618,551. Theimage thereby produced demonstrates the image capability of the process.

EXAMPLE II The process of Example I is repeated excepting a Mylar sheet,polyethylene terephthalate commercially available from El. du Pont deNemours and Company, is substituted for the electrographic paper.Results similar to those obtained in Example I are realized.

EXAMPLE III A plate of anodized aluminum is dip coated with a Staybeliteresin to a thickness of about 10 microns. The coated aluminum plate iscorona charged to a potential of The process of Example 111 is repeatedexcepting Mylar is substituted for the Staybelite resin. Similar resultsare obtained. v

EXAMPLE V The process of Example I is repeated excepting a kryptonfilled flash lamp commercially available from Edgerton, Ger-meshausenand'Grier, 'Inc. is substituted for the xenon flash lamp- The remainingsteps of the process being the same, the results, vas obtained'inExample I are duplicated.

EXAMPLE v1 The process of Example I is repeated excepting a PEK 75 wattxenon arc-lamp (X75) is substituted for the flash lamp. The remainingsteps ofthe procedure are the same as those disclosed in Example I. Theexposure time was increased so as to expose the 'plate to an equivalentamount of energyas provided by the flash lamp. An electrostaticlatentimage is produced and the image developed as described in ExampleI. However, an exposure time of about 30 minutes is necessarytherebyillustrating the fact that a continuous system, as-opposed to-a pulse orflash exposure,

becomes impractical.

Although the present examples were specific in terms of conditions andmaterials used, any of the above listed typical materials may besubstituted when suitable in the above examples with similar results. Inaddition to the steps used in the imaging system of the presentinvention, other steps .or modifications may be used, if desirable. Forexample, the latent image produced in the course of the process may bedeveloped so as to be used as a projection system by heating the platethereby producing a frosted image in the charge retention areas. Inaddition, other materials may be incorporated in the support, dielectricmaterial, developer, or apparatus which will enhance, synergize orotherwise desirably elfect the prop-.- .erties of the imaging system ofthe present invention. For example,-a photosensitive composition may beintroduced into the insulating layer so as to enhance .itsresponsiveness to aparticular range in the electromagnetic radiationspectrum. a 1 Anyone skilled in the art will have other modificationsoccur to him based on the teaching'of the present invention. Thesemodifications are intended to be encompassed 1O withinthe scope-of thisinvention.

-What is claimed is: 1 1. A method of forming an electrostatic latentimage. which comprises coating on the surface of a non-photoconductivesupport substrate a, dielectric material consisting essentially ofpolyethylene terephthalate, applying a uniform electrostatic charge tothe surface of said di-* electric and exposing said charged dielectricto a xenon flash lamp for a period of time ranging from about to about120 microseconds to produce said latent image. 20 2. The method asdisclosed in claim 1 further including the step of developing saidlatent image.

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